... continuing from what Sydian said, a few words of advice to those who want to buy Gen 3 carts online and don't know how to tell the real from the fake (I've had a bad experience myself when a vendor sold me fake GBA carts) :

First of all, the carts themselves. Ruby, Sapphire and Emerald carts are more or less see-through, you should be able to clearly see the PCB (printed circuit board) with good lighting. FireRed and LeafGreen are less so, but aren't completely opaque, you should be able to see the PCB's shape, complete with multiple holes, but that's it.

The labels on the cart must have the Nintendo Seal of Quality, the "The Pokémon Company" written in small black letters, the Nintendo logo, and a code (example: on my French copy of LeafGreen, it's "AGB-BPGF-FRA") , plus other logos that vary by region (example: I have a CE logo on my LeafGreen, AFAIK it only applies to European releases). The label itself must look a little "metallic" and have a two-digit number pressed on it. It is also a good sign if the label is worn-out (which means this is a ten years old cart that was actually used, not a recently manufactured fake) .

On the back of the cart, you should find the Nintendo logo, and two lines of text: "MODEL NO.AGB-002" and "PAT. PEND. MADE IN JAPAN". Some particularly "gifted" bootleggers manage to misspell "Nintendo" on the logo . The screw should have a tri-wing drive.

If you look inside the cart at the bottom, you should be able to see the Nintendo logo printed in white on the PCB.

Although the presence of a battery inside a GBA cart is usually a telltale sign that it's a fake, Ruby, Sapphire and Emerald carts are supposed to have them to keep the internal clock running. FR and LG however are not supposed to have batteries.

Once inserted into a GBA, a GBA SP or a GB Player, the cart must not stick out.

Two definite signs of a fake RSE cart: after the title screen, a text box simply saying "The game can be played" appears, and the internal clock only runs while the game is running (although the latter can occur with the real deal, if the battery is out of juice).

And, just in case someone asks "Why should I care if it's a fake as long as it's working?", here's why:

These carts use batteries to save, just like GB/GBC carts, but unlike the latter, they mostly use cheap batteries. Which means your saved games won't remain there for long.

Also, I've read that some fake carts actually use batteries to save not only the save files, but also the games themselves. If the battery runs out, the cart becomes completely unusable. I can't be sure, but I think that's what happened to a fake FireRed cart I bought: it did not run at all, and my GB Player behaved like no cart was inserted.

Specific to Pokémon: the save file can also get wiped after conquering the league or trying to migrate Pokémon to Gen 4.

One final word of advice: if you live in a country which had localized releases of Gen 3 in a language other than English, you'll be better off ordering the game in your language, since the bootleggers usually pirate English versions.